Vesta possesses features of both asteroids—of which it is one of the largest examples—and of planets. Recent observations by the Dawn mission have provided a detailed map of the asteroid's surface and a great deal of insight into its interior. This data shows Vesta to be part of the rubble of the early Solar System's history, with a structured interior similar to the terrestrial planets—Mercury, Venus, Earth, and Mars. Dawn recently departed Vesta orbit and is now headed to Ceres, the largest asteroid (and one of the five dwarf planets designated by the International Astronomical Union).

Even as Dawn is in transit, scientists are still going through data from its time at Vesta. A pair of studies published in Science reveals a new surprise: the possible presence of volatiles on the asteroid's surface. Additionally, Vesta's surface contains a much higher amount of hydrogen than expected. The asteroid's overall composition indicates that it may be the source of some meteorites found on Earth, and, like Earth, it received volatiles through a late bombardment.

Based on Dawn's observations, Vesta is chemically similar to the HED meteorites, and in fact is likely their source, as material could have been blasted free by collisions. ("HED" comes from three classes of meteorite structure: howardite, eucrite, and diogenite.) However, these meteorites are not high in volatiles, especially compared with the carbonaceous chondrite meteorites thought by many to be the origin of much of Earth's water.

The presence of volatiles on the asteroid—and especially hydrogen—suggests they were added to the asteroid later. This implies a common source for Earth's and Vesta's water, further cementing Vesta's status as a terrestrial world in its own right.

Hydrogen on the rocks

Radioactive decays and reactions from cosmic rays produce gamma rays and neutrons, which can be detected by Dawn provided they originate within a half-meter of the surface, using an instrument called GRaND (Gamma Ray and Neutron Detector). Based on GRaND measurements, researchers mapped the hydrogen content of Vesta's surface. The amount of hydrogen was surprising, since it is the lightest element and therefore extremely volatile. Other airless bodies, such as the Moon, are hydrogen-poor, as are the HED meteorites that originated on Vesta.

However, hints about the hydrogen's origin exist in its distribution. The researchers found hydrogen to be most abundant in regions with low albedo—where little light is reflected. Carbonaceous chondrite meteorites have similarly low albedo, but relatively high water content. This suggests the hydrogen on Vesta's surface came from the dissociation of water deposited by meteorite impacts early in the asteroid's history.

The pits of despair

Further evidence for volatiles came from strange pitted terrain on the otherwise smooth floors of craters. These pits lacked raised rims and were often irregular in shape, meaning they were not impact sites. (Craters are typically rounded with lifted edges.) The pits in the Marcia crater ranged in size from about 30 meters to 250 meters in diameter (though instrument limitations mean there might be smaller ones yet unobserved), with the largest pits occurring near the crater's center.

Pits of this type have never been seen on other airless objects, including Mercury and the Moon. However, they strongly resemble features found in relatively recent craters on Mars. On Mars, the formation mechanism involves volatiles, most likely the vaporization of water ice that collapsed the pits after an impact. This is akin to holes forming in bread dough, when carbon dioxide bubbles burst through the surface during the rising process.

As with the surprising amount of hydrogen, the volatiles could have been deposited by meteorite bombardment after Vesta's formation. The Marcia crater lies in a region low in hydrogen content, which lends further support to this model: as hydrogen is volatile, its leeching from the rocks would be part of the process that created the pits.

This data strongly supports the widely accepted model that Vesta is a planetesimal, a leftover fragment of the bodies that combined to make Earth and other terrestrial worlds. Additionally, the probable presence of water and other volatiles means Vesta's history echoes that of Earth, including the deposition of materials by carbonaceous chondrite meteorites. Further missions to asteroids should settle whether this is a common feature of the larger ones. But for now, the data indicates that Vesta in some ways resembles Earth more closely than it does the Moon.

You begin talking about the Asteroid and the Mission - then throw in a comment about the inner Planets then get to Ceres. Which in turn is the only indication for any context of what the hell you are talkling about. After you have already thrown us a curve.

ardent wrote:

Vesta is a large asteroid within the Solar system's asteroid belt. The Dawn program is exploring several of the larger asteroids.

Would you like to know more?

We all clued into that - not the point - that information should be disseminated in the article.

For what its worth, I had no trouble figuring out what the article was about, but then I am fairly up on current space missions and astronomy in general. That said, I dont see the article as being any more cryptic (to casual audiences) than any of the tech or particle physics articles posted here. IMO, an author writing for this audience should assume some basic science/tech background for the reader, which this author did.

If you are not up on the specific topic, as always, google is your friend.

For what its worth, I had no trouble figuring out what the article was about, but then I am fairly up on current space missions and astronomy in general. That said, I dont see the article as being any more cryptic (to casual audiences) than any of the tech or particle physics articles posted here. IMO, an author writing for this audience should assume some basic science/tech background for the reader, which this author did.

If you are not up on the specific topic, as always, google is your friend.

That makes you the exception that proves the rule.

I'm not asking for dumbing down of articles just some basic frame of reference via previous links or wiki articles, or a single paragraph explaining the Dawn mission to giant planetoids.

I shouldn't have to pull up 3 wiki pages to figure out what the hell an article is talking about. That is all.

This isn't the first article on Ars about the Dawn mission, or Vesta, is it? I could swear we've had multiple articles about this before. No mystery to me what the article was about.

Some searching managed to find those, yes. But assuming that everyone that visits ars reads every single article is a bit... arrogant? Is that the right word?

No.

I dunno; it just wasn't that obscure to me, largely because of what I've read here. Maybe I just gravitate towards those kinds of articles more than others do, or retain that kind of information better. God knows most of the tech policy articles put me right to sleep, so that may be it.

Hyperlinking to the wiki page for Vesta in the very first word would have quieted down the rabble ( or perhaps a brief one sentence descriptioin on what it is along with the link). As it is people have a habit of making a mountain out of a mole hill. I don't understand quite a bit of the technical talk on some of the programming articles but I have no issue searching while I read. Can't please everone I guess.

On topic, Ceres is what I'm excited about. We'll get to see what a failed planet looks like. I can't wait.

The point is the article should have enough info where that isn't necessary except to gain a deeper understanding beyond the basics, which felt lacking.

While I am not generally an "Ars basher" (I love the coverage, generally) and I also had no trouble at all with the topic, I agree with MGT that the article would have benefited from at least a link to a Wiki page about the Dawn mission and/or Vesta itself.

I liked the article. It presumed a basic amount of knowledge - honestly not a big ask, Vesta and Dawn are both farily well known - and cuts to the meat of the issue. More facts, less hand holding. Keep up the good work!

However, these meteorites are not high in volatiles, especially compared with the carbonaceous chondrite meteorites thought by many to be the origin of much of Earth's water.

The presence of volatiles on the asteroid—and especially hydrogen—suggests they were added to the asteroid later.

Terrestrial planets better not have high volatile content, or they would be water worlds. The prediction is that the mantle will have as much water as the oceans, and since Earth's oceans are ~ 0.023 % water per mass the whole Earth has ~ 0.05 % water by mass. As a comparison carbonaceous chondrites may have ~ 10 % water by mass.

A recent assessment of the martian meteorites predict that Mars had the same initial mantle water content as Earth's and Moon's mantles.

Coincidentally, a reassessment of modern protoplanetary disk formation models naturally predicts the required dry zone and outer snow line for Earth to be consistent with the observed water content. The dry zone happens to extend to all terrestrials of the solar system. [ http://astrobites.com/2012/07/25/snow-l ... -water-go/ ]

Quote:

This paper aims to address this discrepancy by addressing a particular assumption made by previous work: that the protoplanetary disk is turbulent, and that the magnetic-rotational instability (MRI) is responsible for this turbulence. However, this instability requires the disk to be ionized. While solar irradiation can ionize the disk very close to the star, further away cosmic rays are required for ionization – and if the disk is thick enough, then they can only ionize the surface. This can lead to a nonturbulent “dead zone” near the disk midplane.

Quote:

By contrast, the model that includes a turbulence-free dead zone has an inner icy region and a central ice-free region. The latter is attributed to a small amount of turbulence due to self-gravity that heats the outer region of the dead zone.

This is very exciting – after t~1 million years, there is a growing ice-free region right around the Earth’s orbit! This resolves the discrepancy of previous models, and provides ample time for an ice-free Earth to evolve. Further work will be necessary to validate this model. If it proves consistent, then we may have reconciled planet formation theory with the water-poor Earth: Regions of low turbulence in the protoplanetary disk allow formation of water-poor terrestrial planets.

So our terrestrials's water content may be naturally predicted by disk formation. The remaining problem is that D/H isotope ratios may be inconsistent, and that perhaps asteroids did contribute some water. Also, the Earth-Moon impact event may have lost volatiles. How much finetuning was involved for Earth and Moon would be hard to easily estimate.

More interesting would be the generic process for thick protoplanetary disks. They would naturally make habitable terrestrials (at least for sun-like stars) dry enough for habitability as we know it: some oceans, some continents.

Could Vesta and Ceres be easier, safer or more interesting destinations for manned spacecraft than Mars? The mineral resources sound adequate for generating fuel for Earth return...

Vesta and Ceres certainly sound like interesting destinations, but they would be more challenging for a manned mission, mostly because they're so much farther away. Someone can correct me with the actual values, but you can think of it this way: flying to the moon takes days, flying to Mars takes weeks and flying to asteroids takes months.

On topic, Ceres is what I'm excited about. We'll get to see what a failed planet looks like. I can't wait.

According to Dawn (and HEDs), already Vesta is a failed planet. Vesta - Ceres - Mars are a related series of planetesimals, fairly rapidly differentiated and too small for long time surface habitability.

In the case of Mars it differentiated in ~ 3 million years, an order of magnitude faster than Earth at ~ 30 million years. Then again, it _is_ an order of magnitude less massive.

I'm sorry. If you want to call Pluto a planet then you should also call Ceres a planet.

If Pluto is a planet you would have to also classify as planets Varuna, Quaoar, Orcus, Ixion, Sedna, Haumea, Eris, Makemake (all named trans-Neptune objects), as well as any other trans-Neptune objects of significant size. The current estimates runs to around 200, if you define a planet as an object with sufficient size and mass to pull itself into a round shape, achieving hydrostatic equilibrium.

The evidence for gas giants (Jupiter, Saturn) and ice giants (Uranus, Neptune) being "successful terrestrials" by gas accreation around a solid core as opposed to brown dwarf analogous gas collapse grows stronger. Brown dwarfs are likely more like "failed stars" (and can even fuse deuterium for a while).

It is Mercury and Mars that go with that criteria. Or perhaps all terrestrials, as they loose atmospheres over billions of years, 10 - 20 billion years for superEarths in some models.

All criteria of populations have this messiness. Planets is a definition that is convenient for astronomers when they want to identify and work with the dominant masses in systems besides the stars themselves.

It is also convenient to inform others on science, which likely was a major reason to keep the planets a distinguished handful. I like it, especially as the "dominant mass" criteria translates very well to the dynamics of planetary systems.

Gas vs ice giants are temporary classifications, roughly like "Jupiters" vs "Neptunes" but less Sun-centric, and not universal. They discuss a lot of classifications for exo-planets, some that you can use from first observations (gas giants, say), some that need a thorough observation. [ http://en.wikipedia.org/wiki/Sudarsky_e ... sification ]